Quote
Accidents WILL and DO happen, people do die in accidents, how do the incidents of injury and possibly death stack up between sailing small boats and driving go carts

We can justify it all we want. Personally, I don't believe that the "more people die in activity X, therefore it's alright that people die in activity Y" rationale is ethical. The fact is that an eleven year old girl needlessly drowned. If the boat had been fitted with a mast float, she would still be alive, doing whatever 14 year old girls do.
Quote

Annetts was the first safety boat employee on the scene. Giving evidence, he said: "When I arrived, the catamaran was on its side and two of the girls were floating in the water."

The twins swam to safety but Laura was unable to free herself. He told the girl to be calm but at that point the catamaran turned upside down, dragging her under. He dived to her rescue.

"I immediately found Laura struggling, panicking to try to get to the surface. I immediately tried to locate the hook and release her and bring her to the surface. I could not release her. She appeared trapped in some other wire."

"Because the visibility was poor and there was lots of struggling and waving around, I could not identify how she was trapped. At that point I had to come back to the surface for air." Mr Annetts radioed for help and dived back under.

"The struggling was continuing and Laura was climbing on me and I could still not release her."

Quote

Since the accident, the RYA has revised its conditions for training centres in an attempt to minimise the risk of “entrapment and entanglement accidents”. Forty-four such incidents were logged during 2003 and 2004, 18 of which were regarded as “very serious, with crew or helm being trapped under water for a period”.

Links: One, two, three, four.

Mast floats clearly save lives, with little downside, why aren't they used?